Diffractive grating element for balancing diffraction efficiency

ABSTRACT

A diffractive grating element is divided into at least two different grating regions each having different diffractive properties and arranged on opposite sides with respect to a transition point to form a splitted grating structure. The diffractions generated by the at least two different grating regions are arranged to mutually compensate for the variation in the input angle of the incident light wave to the total diffraction efficiency of the at least one diffracted light wave that is arranged to propagate within the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS Field of the Invention

This application is the U.S. National Stage of International ApplicationNumber PCT/FI2003/000948 filed Dec. 12, 2003, published in English Jul.1, 2004, under International Publication Number WO 2004/055556 and whichclaims priority to Finnish Patent Application No. 20022199 filed on Dec.16, 2002.

The present invention relates to a diffractive grating element arrangedon or embedded within a light-transmittive, preferably planarwaveguiding substrate and arranged to interact with an incident lightwave in order to couple the energy from said incident light wave intosaid substrate to form at least one diffracted light wave propagatingwithin said substrate and corresponding to at least one selecteddiffraction order.

BACKGROUND OF THE INVENTION

Display technology is one of the key elements in the development of newportable devices, which today typically feature wireless connectivityfor voice and data access—and that will include a display for viewing,for example, text, graphics and different types of multimedia. Thedisplays of such portable devices need to be increasingly capable ofreproducing high quality still images and also live video. Such devicesinclude, for example, advanced mobile phones and portable Internetappliances.

Many portable products are dominated by the display—both physically andfrom the cost perspective. The fact is that almost all other electroniccomponents in such devices are shrinking in size except the display. Theuse of a microdisplay-based system instead of a large size direct viewdisplay panel promises one possible way to get over these limitations.Microdisplay-based systems may be generally defined as systems where theimage produced by an image source has to be magnified for viewing.Generally, such microdisplay-based systems are driven by small,high-resolution integrated circuit display chips, but otherconfigurations are possible too.

Microdisplays offer designers a chance to increase the displayed imagesize and resolution, yet physically shrink the size of the image sourceitself. In many cases, the smaller the image source, the lower the cost.So, not only do microdisplays promise to lower system costs, but theirphysically smaller size will mean less bulky and heavy products andsmaller power consumption, that is they will operate longer with thesame battery source. With a microdisplay-based system high pixeldensities may be achieved. Many direct view flat-panel displays forinstance, produce full colour pixels at only 3-4 lines/mm. Manymicrodisplay-based systems can provide full colour pixels at 50-100lines per mm.

Microdisplay-based systems can be generally divided into two classes:projection display systems and virtual display systems.

Projection display systems create a real image on a screen. Suitableimaging optics magnify and project an image that is created on a displaychip embedded within a projector.

Virtual microdisplay-based systems also use imaging optics to magnify animage, but to create a virtual image instead of a projected real image.A virtual image is what one sees when looking in an electronicviewfinder of a digital video camera, for example. The virtual imageappears to be larger and floating at some distance from the eye of theobserver—even though it is created by a small sized integrated displaychip acting as the image source. In other words, the viewer has theillusion of seeing the source image as if he/she stands a certaindistance away in front of a larger display monitor.

Virtual displays, which are kept close to the eye, can be monocular orbiocular. One type of virtual display is, for example, a Head Up Display(HUD), where the imaging optics are located somewhat further away fromthe eye.

An important and well-known aspect in virtual display devices, as alsoin many other optical systems, is the exit pupil diameter of the system.The diameter and also the location of the exit pupil are of considerablepractical importance defining largely the overall usability of thevirtual display device. In visual instruments, including the virtualdisplays, the observer's eye needs to be positioned at the center of theexit pupil located behind the optical system in order to see the imagewith full field-of-view. In other words, the exit pupil is like awindow, through which the virtual image can be seen.

The distance between the location of the exit pupil and the last opticalcomponent, for example, the eye-piece lens of a visual instrument iscalled eye relief. The eye relief, together with the exit pupil, definesthe freedom of observation, i.e. the volume where the observer's eye canbe located with respect to the optical system.

SUMMARY OF THE INVENTION

The current invention is especially related to such virtual displaysystems, where diffractive grating elements are used as a part of theimaging optics in order to create an enlarged virtual image from thesmaller sized real image created by an image source, herein referred toas an imager, which is typically an integrated circuit display chip. Theinvention is not limited only to microdisplay-based systems, but canalso be used in other virtual display systems. Besides display systems,the invention may in its generic form be utilized in other types ofoptical systems as well, where diffractive grating elements are used forexpanding the exit pupil of the optical system.

The basic use of diffractive grating elements for expanding the exitpupil of an imaging system is already known in the art. For example,patent publication WO 99/52002 discloses optical devices, in which aplurality of holographic optical elements (HOEs), i.e. diffractivegrating elements are arranged on a common planar light-transmittivesubstrate. The disclosed devices may be used for magnifying the exitpupil of the imaging optics, which produces a virtual image at infinityfrom a real image source, and to further reflect this virtual image intothe eye of an observer. The enlargement of the exit pupil of a virtualdisplay system with a beam-expanding optical configuration, such as withthose described in the document WO 99/52002, results in larger eyerelief, which makes the virtual display device more convenient to use. Asignificantly larger eye relief allows to move the display devicefurther away from the immediate vicinity of the observer's eyes. Thismakes it possible to observe the virtual display in a manner resemblingthe use of an ordinary display panel reproducing real images.

Therefore, there is significant interest in optical beam expansionsystems, which can be used to enlarge the diameter of the exit pupil,and further also the eye relief in virtual display systems. Theseoptical beam expansion systems are hereafter referred to as exit pupilextenders (EPEs).

However, prior art solutions for EPEs based on the use ofholographic/diffractive grating elements (HOEs, or DOEs, i.e.diffractive optical elements) have certain significant limitations,which in practice degrade the quality of the reproduced virtual images.One of these limitations is the fact that the diffraction efficiency ofa DOE has rather strong angular dependency. Therefore, when the inputangle of the light incident to an EPE changes, also the spatialdistribution of the light in the EPE changes leading to non-uniformintensity/brightness distribution in the virtual image.

FIG. 1 describes in a very simplified cross-sectional view one possibleconfiguration of a biocular type EPE. DOE1 couples the light from animager into a transparent substrate S, where the light is furtherdiffracted by DOE1 towards left and right directions to be waveguidedalong said substrate S. In the aforementioned directions the lighttravels inside the substrate S based on total internal reflections (TIR)until a second diffractive element DOE2 couples the light out from thesubstrate towards the observer. Separate DOE2s are arranged for the leftand right eye of the observer. FIG. 2 presents another possibleconfiguration for a biocular EPE, where the diffractive elements DOE1and DOE2s are arranged on the upper surface of the substrate S insteadof the lower surface. It is clear for a person skilled in the art thatthe diffractive elements DOE1, DOE2 forming an EPE can be arranged on asubstrate also in certain other ways still maintaining the basicoperation of the device.

FIG. 3 describes schematically the basic problem in EPEs that thecurrent invention primarily aims to solve. A prior art type grating G(corresponding DOE1 in FIGS. 1 and 2) with symmetrical, and in thisexample sinusoidal grating period profile diffracts the incoming lighthaving input angle θ into left and right 1st diffraction orders, markedR⁻¹ and R₊₁, respectively. Here the period of the grating G has beenselected in a manner that, in addition to the 0th order, diffractiontakes place substantially only to left R⁻¹ and right R₊₁ directionscorresponding to the 1st diffraction order. It is evident for a personskilled in the art that when the input angle θ changes, the amount oflight diffracted to the left and right directions along substrate Schanges, i.e. the light is not divided between the directions R⁻¹ andR₊₁ in an equally balanced manner.

FIG. 4 shows calculated angular dependency of diffraction efficienciesat the optimum profile depth of an aluminium coated grating G havingsymmetrical sinusoidal period profile and corresponding to thearrangement shown in FIG. 3. It is clear from FIG. 4 that when the inputangle θ deviates from zero, then also the diffraction efficiencycorresponding to R⁻¹ and R₊₁ changes so that the light will bedistributed non-uniformly between the left and right directions. InFIGS. 3 and 4 the sign of the input angle θ is defined so that therotation of the incoming beam clockwise corresponds to negative inputangles and vice versa.

FIG. 5 describes schematically the possibility to use a so-called blazedgrating BG to affect the distribution of light between the left R⁻¹ andright R₊₁ directions. Blazed gratings, where the profile of each periodof the grating is asymmetrical in a certain way, are well known from therelated art. By designing the grating period to have a suitable degreeof asymmetry (blaze angle), the diffraction efficiency of the blazedgrating can be affected and the diffraction can be concentrated into oneor more selected diffraction orders, i.e. into selected directions.

The main purpose of the current invention is to present a novel solutionfor decreasing or eliminating completely the above explained angulardependency of the diffraction efficiency in a diffractive gratingelement. The invention is especially suitable for beam expansionpurposes for example in EPEs and virtual display devices. A specific aimof the invention is to make possible to construct biocular and alsomonocular virtual display systems, where light can be uniformlydistributed over the whole area of the exit pupil of the display inorder to achieve high image quality.

To attain these purposes, a device comprising a waveguiding substrateand a diffractive grating element according to the invention comprisesat least two different grating regions each having different diffractiveproperties and arranged on opposite sides with respect to a transitionpoint to form a splitted grating element, wherein diffractions generatedby said at least two different grating regions are arranged to mutuallycompensate for an effect of a variation in the input angle of theincident light wave at a given point of the gratings on the totaldiffraction efficiency of the at least one diffracted light wavepropagating within the substrate. The detailed description belowdescribes further some preferred embodiments of the invention.

The basic idea of the invention is to substitute a continuousdiffractive grating element or structure having a grating profileextending in an unchangeably, substantially continuous manner over thewhole area where the incoming beam interacts with said grating, with agrating structure experiencing a transition in the grating profiletypically within said area of interaction. Preferably, in biocularsystems, said transition in the grating profile takes placesubstantially in the center of said area of interaction, i.e.substantially in the location where the optical axis of the incomingbeam passes through the grating. In the following a grating elementaccording to the invention is referred shortly to as a “splittedgrating”. The point where the transition of the grating profile takesplace, i.e. the point where the grating is “splitted” is referred to asthe “transition point”.

According to the invention, the angular dependency of a diffractivegrating can be effectively compensated or eliminated using a splitted,preferably blazed grating together with input optics that have a certainbeam offset as a function of the input angle.

In other words, the diffractive grating element according to theinvention is divided into at least two different grating regions eachhaving different diffractive properties and arranged on opposite sideswith respect to the transition point to form a splitted gratingstructure. The diffractions generated by said at least two differentgrating regions are arranged to mutually compensate for the variation inthe input angle of the incident light wave to the total diffractionefficiency of the at least one diffracted light wave that is arranged topropagate within said substrate.

In biocular systems, according to one preferred embodiment of theinvention, the grating element is arranged to be symmetrically splitted,i.e. the grating period profiles of the two different sides of thegrating are arranged to be substantially mirror images of each otherrespect to transition point. Further, the grating profile in each ofsaid sides is preferably a blazed type profile.

In a monocular system the transition point is substantially outside thebeam area, and the splitting is not symmetrical, as will be shown later.Therefore, the first interaction of the incident beam with the splittedgrating element is arranged to take place substantially within a singlegrating region In monocular systems the less intense diffracted beam,R⁻¹, is recirculated towards the original direction of R₊₁ beam. Theintensity of said right hand side travelling beam is thus increased andits intensity is then substantially independent of incoming angle.

With the invention good image quality with high and even brightness overthe whole exit pupil can be achieved in both monocular or biocular EPEs.One specific object of the invention is thus to allow the manufacture ofvirtual display devices with significantly larger exit pupil diameterthan prior art solutions without degrading the image quality. Along withlarger exit pupil diameters, also a significantly larger eye relief canbe achieved.

The preferred embodiments of the invention and their benefits willbecome more apparent to a person skilled in the art through thedescription and examples given herein below, and also through theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in more detail withreference to the appended drawings, in which

FIG. 1 illustrates schematically in a cross-sectional view one possibleconfiguration of a biocular type EPE,

FIG. 2 illustrates schematically in a cross-sectional view anotherpossible configuration of a biocular type EPE,

FIG. 3 illustrates schematically the basic problem existing in prior artEPEs related to the angular dependency of the diffraction efficiency ofthe grating element,

FIG. 4 shows calculated diffraction efficiency of a of a grating havingsymmetrical sinusoidal period profile and corresponding to thearrangement shown in FIG. 3,

FIG. 5 describes schematically a prior art type use of a blazed gratingin an EPE,

FIG. 6 describes schematically a symmetrically splitted gratingaccording to the invention with asymmetric period profile,

FIG. 7 describes schematically a symmetrically splitted gratingaccording to the invention with beam shift,

FIG. 8 describes schematically one possible optical setup for directingthe light from an imager towards the splitted grating structure wheninput angle θ=0,

FIG. 9 same as FIG. 8 but when input angle θ<>0,

FIG. 10 describes schematically an alternative optical setup fordirecting the light from an imager towards the splitted gratingstructure,

FIG. 11 shows simulation results for total diffraction efficiency of analuminium coated splitted grating,

FIG. 12 shows schematically a monocular EPE based on a splitted gratingaccording to the invention, and

FIG. 13 a,b show coupling efficiencies as a function of grating profiledepth of a blazed grating having period of 440 nm and for wavelength 540nm. FIG. 13 a corresponds to the diffraction of a light beam travellinginside the plate at an angle of 46.4 degrees. FIG. 13 b corresponds todiffraction of a beam entering the substrate plate substantially inperpendicular direction.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the drawings presented herein below aredesigned solely for purposes of illustration and thus, for example, notfor showing the various components of the devices in their correctrelative scale and/or shape. For the sake of clarity, the components anddetails which are not essential in order to explain the spirit of theinvention have been omitted in the drawings.

FIGS. 1-5, which present solutions known from the related art, have beenalready discussed above.

FIG. 6 illustrates schematically a preferred embodiment according to theinvention. A grating profile with asymmetrical period profile, in thisparticular example with a blazed period profile, is splittedsymmetrically with respect to a transition point TP into left BG_(left)and right BG_(right) sides to form a splitted grating element SG. Theleft BG_(left) and right BG_(right) sides of the grating are mirrorimages of each other with respect to said transition point TP. Thetransition point TP is arranged on the point where the optical axis A ofthe incoming beam passes through the grating surface.

Because the wavefront representing the incident beam hits now on bothsides BG_(left) and BG_(right) of the grating element SG, the followingdiffracted beams are generated. R_(−1,left) is generated from the firstorder diffraction towards left from the left side BG_(left) of thegrating. Correspondingly, R_(+1,left) is generated from the first orderdiffraction towards right from the same left side BG_(left) of thegrating. In this example case, because the input angle θ has a negativevalue, R_(+1,left) diffraction is stronger compared to R_(−1,left)diffraction. In similar manner, the right side BG_(right) of the gratingcreates diffraction R_(−1,right) to the left and R_(+1,right) to theright. Now, R_(+1,right) diffraction is weaker compared to R_(−1,right)diffraction. Because of the splitted construction of the grating SG, thetotal diffraction towards left and right are substantially equal:R_(−1,left) summed up together with R_(−1,right) equals substantially inintensity compared to R_(+1,left) summed up together with R_(+1,right).In other words, the different sides BG_(left,) BG_(right) of thesplitted grating element SG are arranged to mutually compensate for thevariation in the input angle (θ) of the incident light wave (W).

FIG. 7 illustrates schematically what happens when the beam incident tothe splitted grating element SG shifts with respect to the transitionpoint TP. The relative amount of diffraction generated by the leftBG_(left) and the right BG_(right) side of the grating changes, butstill R_(−1,left) summed up together with R_(−1,right) equalssubstantially in intensity compared to R_(+1,left) summed up togetherwith R_(+1,right).

FIG. 8 illustrates schematically one possible optical setup fordirecting the light from an imager, for example from a microdisplaychip, towards the splitted grating element SG. FIG. 8 presentsschematically how light from an image point located in the center of theimager surface is directed towards the grating through the ocular typeoptics. Correspondingly, FIG. 9 presents the case where an image pointis located near the edge of the imager surface. It is evident for aperson skilled in-the art that when the image point “moves” from thecenter of the image surface towards the edge of the same, also the inputangle of the light incident on the grating changes together with therelative area of the beam hitting the two regions BG_(left), BG_(right)of the splitted grating element SG. However, according to the inventionthe total diffraction towards left (R_(−1,left)+R_(−1,right)) and right(R_(+1,left)+R_(+1,right)) directions along the substrate S aremaintained substantially equal.

FIG. 10 illustrates an alternative optical setup for directing the lightfrom an imager towards the splitted grating structure BG_(left),BG_(right). By reversing the asymmetric grating period profile, in thiscase by reversing the blaze angle, the ratio of R⁻¹ and R₊₁ diffractionefficiencies becomes inversed. Therefore, for an off-centered imagepoint the beam incident on the grating can be allowed to “cross” thecenter optical axis A and still be generating well balanced diffractiontowards the left and right directions along the substrate S.

FIG. 11 shows simulation results for an aluminium coated splittedgrating element SG having blazed period profile and correspondingbasically to the arrangement shown in FIG. 6. The period profile of thegrating is given byPeriod=A*[sin(2πx/d)+0.25 sin(4πx/d)+0.05 sin(6πx/d)  (1)

-   -   where A=max. height (amplitude) of the grating        -   x=location within a single grating period        -   d=length of a single grating period

The total diffraction efficiency η as a function of the input angle θ iscalculated asη=(0.5−kθ)R ₊₁(−θ)+(0.5+kθ)R ⁻¹(θ)  (2)

In Eq. (2) k is a constant describing how much the beam shifts at agiven angle. For example, it is needed that as the whole beam shiftsonto the left side at extreme angle θ_(max), then k gets a maximum valueof 05/θ_(max). In other cases k gets values smaller than that. If k=0,then the beam is not shifted at all on the grating.

In the case, when the input angle θ=0 then the input beam is locatedsymmetrically with respect to the transition point TP, i.e. the firsthalf of the beam is incident on BG_(left) and the second half of thebeam is incident on B_(right). It is evident from FIG. 8 that thanks tothe splitted grating structure according to the invention, the totaldiffraction efficiency η remains substantially constant independent ofthe input angle θ in a situation where the beam “shifts” along thesplitted grating depending on the location of the image point on theimager surface.

FIG. 12 describes schematically how a splitted grating element SGaccording to the invention can be utilized in a monocular EPE. In FIG.12 the first interaction of the incident light wave (W) with thesplitted grating element SG is arranged to take place substantiallywithin a single grating region M_(right). Here the splitted gratingelement SG comprises on the right side a grating surface MBG_(right)optimized to generate first order diffraction R_(+1,right) towards rightalong substrate S. On the left side, the grating surface MBG_(left) isoptimized to generate a second order diffraction R_(+2,left) towardsright along substrate S. The aforementioned construction provideseffective “recirculation” of the R_(−1,right) diffraction “leaking”undesirably from MBG_(right) towards left along substrate S. Namely,based on Bragg reflection grating surface MBG_(left) diffractsR_(−1,right) back towards right as R_(+2,left). It can be shown thatthis “recirculated” beam R_(+2,left) is completely parallel with respectto the beam R_(+1,right). Therefore, if the input angle θ of the beamincident to the right grating surface MBG_(right) changes, altering theratio of R_(−1,right) and R_(+1,right) reflections and the amount oflight “leaking” towards right along the substrate S, the splittedgrating structure is capable of recirculating the light travelling inthe direction opposite than that desired.

FIGS. 13 a and 13 b show how the coupling/diffraction efficiency of thegrating surfaces MBG_(right) and MBG_(left) shown schematically in FIG.12 depend on the depth of said grating profiles. The period of thegratings is given a value of 440 nm and the calculations have beenperformed for light having wavelength of 540 nm with refractive index of1.71. From FIG. 13 a it can be seen that for the Bragg reflection(46.4°) the efficiency of the left grating MBG_(left) can be optimizedto have almost value of one, i.e. the grating reflects essentially allof the light back towards direction R₊₂. FIG. 13 b corresponds todiffraction of a beam entering the substrate S plate substantially inperpendicular direction, i.e. corresponding MBG_(right) in FIG. 12.

The preferred applications of the invention include different types ofvirtual display devices, where beam expansion in one or more directionsis performed to extend the exit pupil of the display device. In suchdisplay devices the image source can be, for example, a sequential colorLCOS-device (Liquid Crystal On Silicon), an OLED-device (Organic LightEmitting Diode), a MEMS-device (MicroElectroMechanical System) or anyother suitable microdisplay device operating in transmission, reflectionor emission.

While the invention has been shown and described above with respect toselected embodiments of virtual display devices, it should be understoodthat these embodiments are only examples and that a person skilled inthe art could construct other embodiments utilizing technical detailsother than those specifically disclosed herein while still remainingwithin the spirit and scope of the present invention. It shouldtherefore be understood that various omissions and substitutions andchanges in the form and detail of the grating element illustrated, aswell as in the operation of the same, may be made by those skilled inthe art without departing from the spirit of the invention. It is theintention, therefore, to restrict the invention only in the mannerindicated by the scope of the claims appended hereto.

For example, the directions referring to left or right in theexemplified embodiments maybe inverted arranging the correspondinggrating surfaces in an appropriate manner. The grating surfaces may alsobe arranged on different sides (lower,upper) of the substrate S as shownin FIGS. 1 and 2.

The exact grating profiles, grating periods or grating profile depths ofthe splitted grating element SG may be selected according to thespecific application or materials utilized. In the splitted gratingelement, it is possible to use diffractive grating regions where thediffraction efficiency varies locally, for example, by arranging thedepth of the grating region to be different at different local distancesmeasured from the transition point TP.

The change in the grating profile when moving over the transition pointTP may be abrupt, as in the given examples, but it may also arranged totake place in a more smooth manner.

In preferred embodiments of the invention blazed grating profiles areused, but other types of grating profiles may also be used. In asplitted grating structure SG the grating profiles on one or both sidesof the transition point TP may thus be blazed, sinusoidal, heightchanging or of any other suitable type.

The substrate S material may be, for example, glass or plastic or othersuitable light-transmittive material. The grating structure may bearranged substantially on the surface of the substrate or it may also beembedded within the substrate as a buried structure. Preferably, thesubstrate S is planar, but it also possible to use other forms ofsubstrate which offer suitable waveguiding properties. The gratings orother areas of the substrate S may have suitable coatings, for examplealuminium coatings, to enhance the reflectivity. It is also possible touse antireflection coatings if necessary.

The applications of the invention may include, for example, portableInternet appliances, hand-held computers, personal digital assistantdevices (PDAs) advanced mobile phones and other mobile stations, digitalvideo and still cameras, wearable computers, computer game devices,specialized bring-to-the-eye products for viewing different types ofinformation or presentations, and other devices including high qualityvirtual display devices.

The invention may also used in other applications than virtual displays.In principle, the invention is suitable to be used in any applicationwhere optical beam expansion in one or more directions is required.Thus, the invention can be applied to different types of opticalcouplers or other light modulator devices as well.

1. A virtual image display device, comprising an imager for providing animage light wave; and a diffractive grating element, responsive to saidimage light wave, for enlarging an exit pupil of said virtual imagedisplay device for displaying said image light wave as graphics, saiddiffractive grating element in turn comprising a waveguiding substrateand a diffractive grating element arranged on or embedded within saidsubstrate and arranged to interact with said image light wave in orderto couple energy from said image light wave into said substrate to format least one diffracted image light wave propagating within saidsubstrate in a direction of selected diffraction order, said gratingelement comprising at least two different grating regions havingdifferent diffractive properties and arranged on opposite sides withrespect to a transition point, wherein diffractions generated by said atleast two different grating regions are arranged to mutually compensatefor an effect of a variation in input angle of said image light wave ata given point of the grating on a total diffraction efficiency of saidat least one diffracted image light wave propagating within saidsubstrate.
 2. The virtual image display device according to claim 1,wherein a grating profile of at least one of the grating regions has anasymmetric period profile, preferably a blazed period profile.
 3. Thevirtual image display device according to claim 1, wherein said regionsare arranged to be symmetrically splitted, that is, the two differentgrating regions have grating period profiles arranged as substantiallymirror images of each other with respect to a transition point.
 4. Thevirtual image display device according to claim 1, wherein said at leasttwo different grating regions have grating period profiles withsubstantially different depths.
 5. The virtual image display deviceaccording to claim 1, wherein diffraction efficiency of at least one ofthe grating regions is arranged to vary at different local distancesmeasured from the transition point.
 6. The virtual image display deviceaccording to claim 1, wherein the transition point is located within anarea where the image light wave first interacts with the diffractivegrating element.
 7. The virtual image display device according to claim1, wherein a first interaction of the image light wave with thediffractive grating element is arranged to take place substantiallywithin a single grating region.
 8. The virtual image display deviceaccording to claim 7, wherein at least one of the grating regions isarranged to redirect or recirculate the image light wave waveguidedwithin the substrate back towards a reverse direction inside thesubstrate.
 9. A device comprising a waveguiding substrate; an imagerhaving a first location of an image point and a second location of animage point; input optics to direct light from said first location pointtowards said substrate to form a first incident light wave and to directlight from said second location towards said substrate to form a secondincident light wave; and a diffractive grating element arranged tocouple energy of said first incident light wave into said substrate toform first diffracted light waves propagating within said substrate in adirection of a first selected diffraction order and to form seconddiffracted light waves propagating within said substrate in a directionof a second selected diffraction order, said diffractive grating elementalso being arranged to couple energy of said second incident light waveinto said substrate to form first diffracted light waves propagatingwithin said substrate in a direction of said first selected diffractionorder and to form second diffracted light waves propagating within saidsubstrate in a direction of said second selected diffraction order,wherein said diffractive grating element comprises at least twodifferent grating regions having different diffractive properties suchthat distribution of light between the direction of said first selecteddiffraction order and the direction of said second selected diffractionorder is arranged to remain substantially the same when light isdirected from said second location instead of light being directed fromsaid first location, wherein said input optics is further arranged toshift said second incident light wave on said grating element withrespect to said first incident light wave.
 10. The device according toclaim 9 wherein said first location is located in the center of asurface of said imager and said second location is located near the edgeof the surface of said imager.
 11. Apparatus comprising waveguidingsubstrate means; imager means having a first location of an image pointand a second location of an image point; input optics means to directlight from said first location towards said substrate means to form afirst incident light wave and to direct light from said location pointtowards said substrate means to form a second incident light wave; and adiffractive grating means arranged to couple energy of said firstincident light wave into said substrate means to form first diffractedlight waves propagating within said substrate means in a direction of afirst selected diffraction order and to form second diffracted lightwaves propagating within said substrate means in a direction of a secondselected diffraction order, said diffractive grating means also beingarranged to couple energy of said second incident light wave into saidsubstrate means to form first diffracted light waves propagating withinsaid substrate means in a direction of said first selected diffractionorder and to form second diffracted light waves propagating within saidsubstrate in a direction of said second selected diffraction order,wherein said diffractive grating means comprises at least two differentgrating regions having different diffractive properties such thatdistribution of light between the direction of said first selecteddiffraction order and the direction of said second selected diffractionorder is arranged to remain substantially the same when light isdirected from said second location instead of light being directed fromsaid first location wherein said input optics is further arranged toshift said second incident light wave on said diffractive grating meanswith respect to said first incident light wave.
 12. The apparatus ofclaim 11 wherein said first location is located in the center of asurface of said imager and said second location is located near the edgeof the surface of said imager.